/* Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 *     http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package java.nio;

/**
 * A buffer is a list of elements of a specific primitive type.
 * <p>
 * A buffer can be described by the following properties:
 * <ul>
 * <li>Capacity: the number of elements a buffer can hold. Capacity may not be
 * negative and never changes.</li>
 * <li>Position: a cursor of this buffer. Elements are read or written at the
 * position if you do not specify an index explicitly. Position may not be
 * negative and not greater than the limit.</li>
 * <li>Limit: controls the scope of accessible elements. You can only read or
 * write elements from index zero to <code>limit - 1</code>. Accessing
 * elements out of the scope will cause an exception. Limit may not be negative
 * and not greater than capacity.</li>
 * <li>Mark: used to remember the current position, so that you can reset the
 * position later. Mark may not be negative and no greater than position.</li>
 * <li>A buffer can be read-only or read-write. Trying to modify the elements
 * of a read-only buffer will cause a <code>ReadOnlyBufferException</code>,
 * while changing the position, limit and mark of a read-only buffer is OK.</li>
 * <li>A buffer can be direct or indirect. A direct buffer will try its best to
 * take advantage of native memory APIs and it may not stay in the Java heap,
 * thus it is not affected by garbage collection.</li>
 * </ul>
 * <p>
 * Buffers are not thread-safe. If concurrent access to a buffer instance is
 * required, then the callers are responsible to take care of the
 * synchronization issues.
 */
public abstract class Buffer {

    /**
     * <code>UNSET_MARK</code> means the mark has not been set.
     */
    final static int UNSET_MARK = -1;

    /**
     * The capacity of this buffer, which never change.
     */
    final int capacity;

    /**
     * <code>limit - 1</code> is the last element that can be read or written.
     * Limit must be no less than zero and no greater than <code>capacity</code>.
     */
    int limit;

    /**
     * Mark is where position will be set when <code>reset()</code> is called.
     * Mark is not set by default. Mark is always no less than zero and no
     * greater than <code>position</code>.
     */
    int mark = UNSET_MARK;

    /**
     * The current position of this buffer. Position is always no less than zero
     * and no greater than <code>limit</code>.
     */
    int position = 0;

    /**
     * Construct a buffer with the specified capacity.
     * 
     * @param capacity
     *            The capacity of this buffer
     */
    Buffer(int capacity) {
        super();
        if (capacity < 0) {
            throw new IllegalArgumentException();
        }
        this.capacity = this.limit = capacity;
    }

	
	/**
	 * Answers the array that backs this buffer.
	 * 
	 * It wants to allow array-backed buffers to be passed to native code more
	 * efficiently. Subclasses provide more concrete return values for this
	 * method.
	 * 
	 * Modifications to this buffer's content will cause the returned array's
	 * content to be modified, and vice versa.
	 * 
	 * Invoking the <code>hasArray</code> method before invoking this method
	 * can guarantee it to be called safely.
	 * 
	 * @return The array that backs this buffer
	 * 
	 * @throws ReadOnlyBufferException -
	 *             If this buffer is backed by an array but is read-only
	 *         UnsupportedOperationException - If this buffer is not backed
	 *             by an accessible array
	 * @since 1.6
	 * 
	 */
	public abstract Object array();
	
	/**
	 * Returns the offset within this buffer's backing array of the first
	 * element of the buffer (optional operation).
	 * 
	 * If this buffer is backed by an array then buffer position p corresponds
	 * to array index p + arrayOffset().
	 * 
	 * Invoke the hasArray method before invoking this method in order to ensure
	 * that this buffer has an accessible backing array.
	 * 
	 * @return The offset within this buffer's array of the first element of the
	 *         buffer
	 * 
	 * @throws ReadOnlyBufferException -
	 *             If this buffer is backed by an array but is read-only
	 *         UnsupportedOperationException - If this buffer is not backed
	 *             by an accessible array
	 * 
	 * @since 1.6
	 */
	public abstract int arrayOffset();

    /**
     * Returns the capacity of this buffer.
     * 
     * @return the number of elements that are contained in this buffer.
     */
    public final int capacity() {
        return capacity;
    }

    /**
     * Clears this buffer.
     * <p>
     * While the content of this buffer is not changed, the following internal
     * changes take place: the current position is reset back to the start of
     * the buffer, the value of the buffer limit is made equal to the capacity
     * and mark is cleared.
     *
     * @return this buffer.
     */
    public final Buffer clear() {
        position = 0;
        mark = UNSET_MARK;
        limit = capacity;
        return this;
    }

    /**
     * Flips this buffer.
     * <p>
     * The limit is set to the current position, then the position is set to
     * zero, and the mark is cleared.
     * <p>
     * The content of this buffer is not changed.
     *
     * @return this buffer.
     */
    public final Buffer flip() {
        limit = position;
        position = 0;
        mark = UNSET_MARK;
        return this;
    }


	
	/**
	 * Answers if this buffer is backed by an available array.
	 * 
	 * If it returns true then the <code>array</code> and
	 * <code>arrayOffset</code> methods can be called safely.
	 * 
	 * @return true if and only if this buffer is backed by an array and is
	 *         not read-only.
	 *         
	 * @since 1.6
	 */
	public abstract boolean hasArray();

    /**
     * Indicates if there are elements remaining in this buffer, that is if
     * {@code position < limit}.
     * 
     * @return {@code true} if there are elements remaining in this buffer,
     *         {@code false} otherwise.
     */
    public final boolean hasRemaining() {
        return position < limit;
    }
	
	/**
	 * Answers if this buffer is direct.
	 * 
	 * @return true if and only if this buffer is direct
	 * 
	 * @since 1.6
	 */
	public abstract boolean isDirect();

    /**
     * Indicates whether this buffer is read-only.
     * 
     * @return {@code true} if this buffer is read-only, {@code false}
     *         otherwise.
     */
    public abstract boolean isReadOnly();

    /**
     * Returns the limit of this buffer.
     * 
     * @return the limit of this buffer.
     */
    public final int limit() {
        return limit;
    }

    /**
     * Sets the limit of this buffer.
     * <p>
     * If the current position in the buffer is in excess of
     * <code>newLimit</code> then, on returning from this call, it will have
     * been adjusted to be equivalent to <code>newLimit</code>. If the mark
     * is set and is greater than the new limit, then it is cleared.
     *
     * @param newLimit
     *            the new limit, must not be negative and not greater than
     *            capacity.
     * @return this buffer.
     * @exception IllegalArgumentException
     *                if <code>newLimit</code> is invalid.
     */
    public final Buffer limit(int newLimit) {
        if (newLimit < 0 || newLimit > capacity) {
            throw new IllegalArgumentException();
        }

        limit = newLimit;
        if (position > newLimit) {
            position = newLimit;
        }
        if ((mark != UNSET_MARK) && (mark > newLimit)) {
            mark = UNSET_MARK;
        }
        return this;
    }

    /**
     * Marks the current position, so that the position may return to this point
     * later by calling <code>reset()</code>.
     * 
     * @return this buffer.
     */
    public final Buffer mark() {
        mark = position;
        return this;
    }

    /**
     * Returns the position of this buffer.
     * 
     * @return the value of this buffer's current position.
     */
    public final int position() {
        return position;
    }

    /**
     * Sets the position of this buffer.
     * <p>
     * If the mark is set and it is greater than the new position, then it is
     * cleared.
     *
     * @param newPosition
     *            the new position, must be not negative and not greater than
     *            limit.
     * @return this buffer.
     * @exception IllegalArgumentException
     *                if <code>newPosition</code> is invalid.
     */
    public final Buffer position(int newPosition) {
        if (newPosition < 0 || newPosition > limit) {
            throw new IllegalArgumentException();
        }

        position = newPosition;
        if ((mark != UNSET_MARK) && (mark > position)) {
            mark = UNSET_MARK;
        }
        return this;
    }

    /**
     * Returns the number of remaining elements in this buffer, that is
     * {@code limit - position}.
     * 
     * @return the number of remaining elements in this buffer.
     */
    public final int remaining() {
        return limit - position;
    }

    /**
     * Resets the position of this buffer to the <code>mark</code>.
     * 
     * @return this buffer.
     * @exception InvalidMarkException
     *                if the mark is not set.
     */
    public final Buffer reset() {
        if (mark == UNSET_MARK) {
            throw new InvalidMarkException();
        }
        position = mark;
        return this;
    }

    /**
     * Rewinds this buffer.
     * <p>
     * The position is set to zero, and the mark is cleared. The content of this
     * buffer is not changed.
     *
     * @return this buffer.
     */
    public final Buffer rewind() {
        position = 0;
        mark = UNSET_MARK;
        return this;
    }
}
